Equipment for supplying molten metal to a continuous casting ingot mould and method for using same

ABSTRACT

The apparatus comprises a submerged entry nozzle ( 6 ) having outlets in the main casting plane (P) which differ in their direction of output and fall within two categories ( 7, 8 ), said nozzle being associated with two inductors ( 14, 15 ) opposite each other on each broad face ( 22 ) of the casting mold forming a gap which surrounds the nozzle and producing a traversing magnetic field covering the outlets of at least one category ( 7 ), means being provided for adjusting the intensity of the field or for moving it so as to be able to change the distribution between the outlets of the total flow of molten metal.  
     In particular, implementing the invention makes it possible to adjust at any time that fraction of the metal flow which is directed toward the free surface ( 9 ) with respect to that, main, fraction directed toward the bottom of the mold.  
     Advantageously, the invention applies to the continuous casting of steel slabs.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to the continuous casting ofmetals, especially steel. It relates more particularly to the supply ofmolten metal from above into a continuous casting mold and even morespecifically to the techniques using magnetic fields applied to the moldin order to modify the flows of molten metal as it enters the mold.

DESRIPTION OF THE PRIOR ART

[0002] It is known that applying a magnetic field to a continuouscasting mold, when the electromagnetic action is performed in a suitablemanner, makes it possible to increase the productivity of the castingplant while still maintaining the metallurgical quality of the castproducts obtained, or even improving it. In this regard, it has alreadybeen demonstrated that, when the casting rate is increased, especiallyin the case of casting products of elongate cross section, such asslabs, the hydrodynamic turbulence due to recirculating flows whichbecome established with increasing strength within the mold is anuisance.

[0003] It will be recalled that, in the continuous casting of slabs, themolten metal is fed into the mold from a tundish placed at a certaindistance above it via a dip pipe, called a “submerged entry nozzle”, theoutlets of which open substantially in the main casting plane parallelto the broad faces below the free surface of the molten steel in themold, said surface being conventionally covered with a liquid layer ofactive slag.

[0004] It has been established that the velocity of the streams ofliquid metal leaving the outlets of the nozzle increases to severalmeters per second as soon as the casting speed reaches about 1 to 1.5m/min. The recirculating flows in the mold which result therefromvigorously stir the metal/slag interface. These fluctuations in the freesurface of the cast metal are responsible for irregularities in thesolidification of the initial shell of the cast product which is knownto be the source of problematic, or indeed unacceptable, defects in thefinal product (blistering, exfoliation, etc.). In addition, fragments ofcovering slag may be taken away in the mold into the very core of thecast product, thus degrading the cleanliness of the solidified metalobtained.

[0005] Faced with the problem posed by these hydrodynamic perturbations,a steelmaker today has at his disposal essentially two types ofsolution, one making use of the available magnetohydrodynamic toolssuitable for the continuous casting of metals and the other relying onthe actual geometry of the casting nozzle.

[0006] The electromagnetic actuators that have been developed for thispurpose, whether with a static or traveling magnetic field, have aninfluence on the recirculating flows of liquid metal in the mold afterit has left the nozzle, so as to brake or accelerate them, or to makethem symmetrical on either side of a submerged entry nozzle.

[0007] Thus, electromagnetic brakes were originally developed thatconsisted in applying, at a predetermined height level in the internalspace of the mold, a traversing magnetic field which creates brakingforces (Laplace forces) in the moving metal when it passes through thisregion. For this purpose, it has been proposed to use, on each broadface of the mold, a magnetic pole, designed like a coiled salient-poleelectromagnet, having the shape either of a protrusion located on eachside of the nozzle between the latter and the narrow end faces of themold (EP-A-0040383), or a horizontal bar extending over the entire widthof the broad face (WO 92/12814) or of two parallel bars spaced apartover the height so as to flank the outlets of the nozzle (WO 96/26029and WO 98/53936). Whatever the geometry adopted, the aim is the same: onthe one hand, to create, with the like pole of opposite sign placedopposite it on the other face of the mold, a traversing magnetic fieldwhose effect is to brake the excessively energetic streams which risetoward the free surface and, on the other hand, to better distributeover the entire cross section of the mold the main stream of liquidmetal which flows downward.

[0008] In order to achieve with this type of technique greater controlflexibility, it has been proposed to use magnetic fields that are nolonger static but traveling, it being known that these have the abilityto entrain liquid metal in their movement (EP-A-0 151 648, WO 83/02079and JP-B-1 534 702). Two inductors with a horizontally traveling field(vertically oriented conductors) are placed on each broad face of themold on each side of a submerged entry nozzle having lateral outlets,between the nozzle and the narrow end faces so as to make the travelingmagnetic field intercept the molten metal as soon as it enters theseregions of the mold. Thus, it is possible to accelerate (or to brake,depending on the direction of relative movement given to the travelingfield) the streams of liquid metal feeding the mold by having theability to locally control the electromagnetic action by simplyadjusting the operating parameters of the inductors, such as, forexample, the intensity of the primary supply electric current, or theangular frequency, and hence the speed of travel of the magnetic field.

[0009] It will be recalled if required that such a traveling magneticfield is generally produced by an inductor having several independentphase windings, of the “polyphase linear motor stator” type (generallytwo-phase or three-phase type) and that this is placed opposite a broadface of the mold, and therefore parallel to the main casting plane(FR-A-2,324,395 and FR-A-2,324,397). Each winding is connected to adifferent phase of a polyphase electrical supply, in a suitableconnection order ensuring that the magnetic field travels in the desiredmanner along the active face of the inductor in a directionperpendicular to the conductors.

[0010] It has also already been proposed, for the purpose this time ofcounteracting the observed phenomena of wave propagation on the freesurface from one narrow face of the mold to the other, to improve thesymmetry of the flows of molten metal entering the mold in the regionson each side of the nozzle by means of a movable magnetic pole, theposition of which can be adjusted mechanically, or of two adjacent fixedmagnetic poles which are intercorrelated in their respective actions onthe moving metal (EP-A-0,832,704 and JP-A-03275256).

[0011] The other type of solution consists in optimizing the geometry ofthe submerged part of the teeming nozzles, especially the outlets forthe molten metal. The aim is always the same, namely to control thedistribution of the flows of liquid metal entering the mold.

[0012] For example, this type of solution includes nozzles of the “box”type (U.S. Pat. No. 464,698 [lacuna] and JP-A-63,76753), the submergedpart of which has an overall bulbous shape reminiscent of a decorator'sbrush or of a flattened sprayhead, the function of which is assumedmoreover to be similar.

[0013] These nozzles are quite extensively open toward the bottom inorder to favor outflow in the main casting plane of the casting streamswith a low velocity but over a large flow section. Their main propertyis thus to try to deliver liquid metal to the mold with a uniform flow,approaching the ideal flow called “plug” flow, in which the velocitygradient between any two points of a cross section is close to zero andsaid section rapidly becomes as close as possible to that of the mold.These box-shaped nozzles are starting to be widely used in the industry,especially on thin-slab continuous casting plants. The recirculatingstreams of metal flowing toward the free surface of the cast metal maythus be highly attenuated, to such a point that it might be possible toprovide, where appropriate, additional openings at the top of the box oralong the side in order to allow streams of molten metal to flow outupward in order to provide an additional uniform supply of heat to thefree surface, which it is known is necessary for the casting to proceedproperly.

[0014] Also within this type of solution are straight nozzles having twodifferent pairs of lateral outlets which are oriented in the maincasting plane, parallel to the broad faces of the mold. Outlets placedin the bottom position on the shaft of the nozzle deliver, generally ina downward direction, the primary stream of metal to be withdrawn fromthe mold. The other outlets are arranged in the top part so as todeliver a secondary stream intended to supply the free surface with heatvia a uniform but low-flowrate supply of “fresh” molten metal that hasonly just entered the mold, and therefore with a high enthalpy. Therelatively low manufacturing cost of this type of nozzle may be asignificant economic advantage in the case of wear components of thiskind, which have to be regularly replaced.

[0015] That being so, whatever the conformation used for thenozzle—straight or box-shaped, it is necessarily fixed in its geometryand therefore can be optimized only for a single method of carrying outthe casting operation, or for a particular shape of cast product. Thistype of approach therefore seems to be ill-suited to the inevitableoperating variations or modifications, whether unintentional orintentional, specific to modern continuous casters, such as variationsin the casting speed, changes in product shape, etc.

[0016] Electromagnetic actuators (brakes, accelerators, symmetrizers)are by nature more flexible to use, and therefore more appropriate forfollowing such variations. However, they are not optimized for anyparticular operating mode. They control the flows of liquid metal onceit has entered the mold and then act sometimes as an accelerator andsometimes as a flow brake. However, they absolutely do not have thecapability, unlike certain of the abovementioned nozzles, to distributethe inflow of molten metal between the top region of the mold (towardthe free surface) and the bottom (in the direction of extraction of thecast product). Furthermore, they are relatively expensive in terms ofinvestment cost and in cost of electrical energy consumption, and theyinvolve complex and financially burdensome modifications in thetechnology of the molds which receive them.

SUMMARY OF THE INVENTION

[0017] The object of the present invention is specifically to providesteelmakers with a means of feeding a continuous casting mold withmolten metal, which readily allows rapid and precise control of theincoming metal flow distribution between the top and bottom regions ofthe mold.

[0018] With this objective in mind, the subject of the invention is anapparatus for supplying a mold of a plant for the continuous casting ofproducts of rectangular cross section, such as slabs, with molten metal,which comprises:

[0019] a submerged entry nozzle provided with outlets for the moltenmetal which lie in, or substantially in, the main casting plane parallelto the broad faces of the mold, these outlets differing in theirdirection of outflow and falling within at least two separate types;

[0020] an inductive unit placed over the broad faces of the mold inorder to produce thereon magnetic poles of opposite sign facing eachother on each side of said main casting plane and delivering, in its gapsubstantially surrounding the nozzle, a traversing magnetic fieldcovering the outlets of at least one of said types; and

[0021] means for adjusting the relative intensity of said magneticfield, in the region of the outlets of said type covered which is, withrespect to the outlets of the other type, so as to be able to modify thedistribution of the total flow of molten metal between all the outletsof said nozzle.

[0022] According to one embodiment, said inductive unit is anelectromagnetic unit consisting of at least one electromagnet.

[0023] According to another embodiment, said inductive unit consists ofinductors having a plurality of phase windings of the “traveling field”type, facing each other on each side of said main casting plane, and ofan associated electrical power supply which supplies each of saidwindings separately with DC current, and the means for adjusting therelative intensity of the magnetic field comprise means for moving thelocation of the magnetic poles in the gap of said electromagnetic unit.

[0024] It is conceivable to use an inductor (an electromagnet or aninductor of the “traveling field” type) only on a single face of themold, but to the detriment in this case of the electromagnetic poweravailable. In any case, according to the invention, the magnetic pole ofthe inductor must always deliver a magnetic field directed perpendicularto the wall of the mold opposite which the inductor is mounted.Otherwise, the desired effect is not obtained. Thus, if two inductorsare face to face, the facing magnetic poles are of opposite sign so asto create a traveling magnetic field, that is to say the lines of forceof which field link the two poles by extending perpendicular to the maincasting plane in which the streams of metal are created through theoutlets of the nozzle placed in the gap between the two inductors.

[0025] A magnetic pole of an inductor is defined as the region of theactive face of the inductor where the magnetic field produced is amaximum. In the case of an electromagnet, the pole is the end, oftenprojecting, of the wound ferromagnetic metal body which characterizesthe device. In the case of an inductor of the traveling-field type witha plurality of phase windings, the magnetic pole does not have a fixedphysical representation attached to a given ferromagnetic body of theyoke, but it can move over the active face of the inductor according tothe instantaneous intensity of the AC phase currents which supply theconductors and according to their phase difference. Likewise, it may besaid that a magnetic field “covers” the nozzle outlets, when the latterlie in a region of space within the mold where the magnetic inductionproduced by this field is a maximum.

[0026] Having been given these details, it will be understood that it iseasy to modify the action of the magnetic field in that region of thenozzle outlets which is covered by this field, according to theinvention (relative to the possible action exerted in the region of theother outlets) by suitably adjusting the intensity of this field in theregion in question. This action is achieved either by varying(decreasing or increasing) the intensity of the magnetic field, withoutmodifying the position of the magnetic pole which delivers it, or bymodifying the position of the magnetic pole on the broad faces of themold while maintaining its intensity. The first operating versionmentioned above may be preferred if, with respect to the size and to thedistance of the magnetic pole used, the outlets of the two types arequite far apart on the body of the nozzle so that the values of themagnetic induction in their respective regions may be very different,while the intensity of the field is a maximum, for example, over theoutlets covered by this field. On the other hand, the second versionmentioned above is better suited to the case, which is doubtlessinevitably the most frequent, in which all the outlets are covered andin which only the movement of the pole can provide a field differentialbetween them which is sufficient to obtain, in a pronounced manner, theresults desired by the invention.

[0027] Of course, in the case of an electromagnet the movement of themagnetic pole will be obtained by mounting the electromagnet so as to beable to move on a frame fastened to the caster and provided with meanswhich make it possible to move it over that face of the mold on which itis mounted and to stop it at the chosen site.

[0028] It is also possible in some cases to benefit by dividing theinductor into two inductive parts placed side by side along the sameface of the mold, each part thus controlling the outlets lying on oneside of the nozzle, independently of those lying on the other side.

[0029] Whatever the embodiment used, it will doubtlessly already havebeen understood that a basic idea of the invention consists in using amagnetic field as a kind of nonphysical valve for closing off thepassage provided by one type of nozzle outlet so as to modify theoutflow from the other type of outlet. Since the feed rate to the nozzleis constant, or in any case hardly affected by the action of themagnetic field, this action, which acts directly at one type of outlet,will have the effect of modifying the distribution of the fractions ofthe total flow between the two types of outlet. What is produced is akind of submerged entry nozzle whose geometry can be varied withoutmodifying its shape.

[0030] Preferably, the main outlets, namely those from which the outflowof molten metal is the greatest, (those generally directed downward)will be covered by the magnetic field since the variations in the actionof this field on the outflows will be more appreciable therein than onthose where the flow of metal is smaller. In the rest of thedescription, it will be assumed for the sake of clarity that themagnetic field covers the main downwardly directed outlets.

[0031] It will also have been understood that, in a preferredembodiment, the invention uses a traversing magnetic field which canmove vertically in the nozzle region but is produced by a fixedinductive unit: a pair of inductors facing each other, each of the“linear motor stator with a traveling magnetic field” type, which arematched so that the inductors are in phase opposition and each of themcan produce a magnetic field whose lines of force are oriented in thesame direction (the condition specific to obtaining a so-called“traversing” magnetic field), but the phase windings of which areconnected to individual DC power supplies that can be adjustedindependently of each other. Such an inductive unit is then capable, asis known, of generating magnetic poles of opposite sign, and therefore atraversing static magnetic field, which can be located at the desiredpoint in the gap. This change in the position of the poles is obtainedby selectively activating the windings of the inductor by simplyadjusting the operating parameters of the individual power supplies,namely, in practice, the intensity of the electric currents which theydeliver. These adjustments can be made instantly, during the actualcasting if so desired, remotely from the caster, completely safely forthe operators, and in a completely transparent manner, that is to saywithout any risk, even minute, of disturbing the proper execution of thecasting operation. It will be recalled that the structure of this typeof inductor has been known for a long time as has indeed its use incontinuous slab casting as a means of moving the molten metal over theheight of the mold (cf. for example the abovementioned patentsFR-A-2,324,395 and FR-A-2,324,397).

[0032] Thus, the subject of the invention is also a process foroperating the preferred apparatus defined above, the process consistingin adjusting the intensity of the magnetic field either by moving theposition of the poles of the inductive unit or by modifying theintensity of the electric current supplying the inductive unit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The invention will be fully understood and further aspects andadvantages will become more clearly apparent in the light of thedescription which follows, given solely by way of illustrativenonlimiting example with reference to the appended plates of drawings,in which:

[0034]FIG. 1 shows schematically, seen from the front, in verticalsection in the main casting plane, a mold for the continuous casting ofsteel slabs provided in its upper part with an apparatus for feedingmolten metal in accordance with the invention in an embodiment with asingle inductor per mold face;

[0035]FIG. 2, as a vignette of FIG. 1, is a diagram explaining thestructure of a flat inductor of known type which may be suitable forimplementing the invention and linked for this purpose to a DCelectrical power supply;

[0036]FIG. 3 is a diagram taken from a vertical cross-sectional view inthe vertical plane R-R of FIG. 1 and illustrating, seen from the side ofthe mold, the “traversing field” operating mode of the invention;

[0037]FIG. 4 is a diagram taken from a horizontal cross-sectional viewin the horizontal plane Q-Q of FIG. 1 and illustrating, seen along thecasting axis, the “traversing field” operating mode of the invention;and

[0038]FIG. 5 is a schematic view similar to that of FIG. 1, butillustrating an embodiment of the invention with two inductors side byside per face of the mold.

[0039] In these figures, the same components are denoted by identicalreference numbers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] A mold 1, made of copper or a copper alloy and vigorously cooledby a circulation of water around its external wall, receives, from thetop, a certain flow of molten metal 2 which it withdraws downward in theform of a semifinished iron or steel product 3, which will be assumedhere to be a steel slab. On leaving the mold, the slab 3, still liquidin the core 4 but already solidified around the periphery 5 as a resultof it coming into contact with the cooled internal wall of the mold,completes its solidification as it advances along the casting axis Sthrough the lower stages of the casting plant, especially by water beingsprayed directly onto its surface. The influx of “fresh” metal into themold takes place via a submerged entry nozzle 6 whose upper part, notvisible in the figure, is fixed around a taphole made in the bottom of atundish placed at a certain distance above it and whose bottom part isimmersed in the mold. This lower part comprises outlets 7, 8 opening outbelow the free surface 9 of the liquid metal covered by a blanket 10 ofcover slag. As may be seen, these outlets, oriented in the main castingplane, are of two different types:

[0041] main outlets 7 inclined downward and delivering the major part ofthe flow of steel feeding the mold by means of streams 11 in an overalldirection lying in the main casting plane (the plane of the figure) andgenerally going toward the bottom of the mold;

[0042] secondary outlets 8 lying above, inclined upward and delivering,somewhat in this direction, the rest of the flow of metal by means ofstreams 12 taking to the surface 9 an influx of heat needed to preventparasitic solidification phenomena on the meniscus (solidificationhooks, etc.).

[0043] The reader is reminded that the expression “main casting plane”is understood to mean the vertical mid-plane P passing through thecasting axis S at the center of the mold and parallel to the broad faces22 of the latter. In this case, FIGS. 1 and 5 lie precisely in the maincasting plane P. The other plane, analogous but parallel to the narrowside faces 13 of the mold, is termed the secondary casting plane. FIGS.3a and 3 b are in the secondary casting plane.

[0044] The law of conservation of “matter” flow means, of course, theflow of metal withdrawn via the bottom of the mold is equal to the flowof metal, entirely liquid, entering the mold via the nozzle 6. Since thespeed of withdrawal V is a casting parameter, it is this speed which,for a given cross section of product 3, determines the incoming flowrateand hence the rate of outflow of liquid metal from the nozzle outlets.As already stated, if the casting plant is a high-productivity plant(speed of withdrawal V threshold of about 1.5 m/min), the recirculatingstreams, which are inevitably set up in the mold because of themagnitude of the difference between the speed of extraction and thespeed, a hundred times greater, of the streams of metal output by thenozzle outlets, quickly become very vigorous. Violent and turbulentrecirculation loops, doped by the reflections of the streams of metaloff the narrow faces 13 of the mold, therefore greatly disturb the freesurface 9. These disturbances are deleterious and must be attenuated, orindeed eliminated. However, this attenuation must not prejudice the heatinflux to the free surface 9 carried by the secondary streams 12. Sincethe operating regime of a continuous caster is above all of the“transient” type, especially because of the variations in the castingspeed, this desired balance between the need for a flat and calm freesurface and for a free surface heated by the “fresh” molten metal comingfrom the nozzle is therefore almost permanently thrown into question.

[0045] This is the reason why, according to the invention, on each broadface 22 of the mold, an inductive unit, consisting of a pair ofelectromagnetic inductors 14, 15, is placed opposite the terminal partof the nozzle. These two inductors are matched so that each produces amagnetic pole facing each other, of opposite sign, so as to create atraversing magnetic field perpendicular to the broad faces 22. As may beseen in FIGS. 1 and 3, this traversing field is located at “M” in thebottom part of the gap so as to “cover” the outlets of type 7 situatedat the bottom end of the body of the nozzle 6. However, these inductorsare designed so that their magnetic poles can be moved together in thegap. Here, the movement will be vertically along the mold since theconductors 16 . . . 17′ lie in the horizontal. This combined movement ofthe poles of the inductor, over a distance of about 10 or 15 cm, willcause a corresponding movement of the traversing magnetic field in thegap, and hence a correlative modification of the local magneticconditions in the region of the different outlets 7 and 8 of the nozzle.Consequently there is a desired redistribution of the flows of metalleaving these two types of outlet, the total flow itself remainingunchanged or almost unchanged. Thus, in FIG. 3, M represents an initialbottom position of the magnetic field in the gap and N represents a topfinal position after vertical movement over a distance “d” in thedirection of the outlets 8 delivering upward streams of metal.

[0046] The movement of the magnetic field may be obtained by means of apair of “electromagnet”-type inductors which are therefore provided witha salient magnetic pole, serving as a support for a wire conductor woundaround it, and are mounted so as to move translationally along a framefastened to the casting plant. This construction therefore requires theinductive unit to physically move.

[0047] When the prevalent conditions so allow, it will be preferable toopt for a magnetic field that can move in a fixed gap. It is known thatsuch a possibility is provided by an inductive unit, such as that shownschematically in FIG. 2, consisting, opposite each other and on eachside of the broad faces 22 of the mold, of two “traveling magneticfield”-type inductors with a plurality of phase windings. The inductorshown here is a flat inductor of the “linear motor stator” type and hastwo phases (and therefore two phase windings). These conductors arestraight copper bars 16, 17, 16′, 17′, four in number, mutuallyparallel, spaced apart and laying horizontally. Each winding is composedof two bars linked together in series opposition so that the electriccurrent flows through them in opposite directions. It does not matterwhether the linked bars are immediately adjacent bars, such as 17 with16′ and 16 with 17′ (inductor with adjacent poles), or are offset, suchas 16 with 16′ and 17 with 17′ (inductor with distributed poles), asshown in the figure.

[0048] However, it is important that, whatever the configuration chosen,each phase winding be connected to an individual DC (or rectified) powersupply and to this power supply alone and which is independent of thatof the other winding. These individual power supplies, shownsymbolically at 18 and 19 in FIG. 2, may have, for reasons ofconvenience, their neutral commoned. They may be integrated into a powersupply unit 20 provided with means 21 a and 21 b for autonomouslyadjusting the intensities of the currents delivered by each individualpower supply 18, 19 so as to be able, for example, to make a current ofmaximum intensity flow in one winding while the other is deactivated(zero current), and vice versa, together with all the intermediateadjustments. It is under these conditions that the flat inductor 14 (15)can create, no longer a traveling field, as is ordinarily the case, buta static magnetic field whose magnetic pole which delivers it can beshifted over the active face of the inductor in a directionperpendicular to the conductors, simply by suitably modifying theintensities of the current in the two windings. A more detaileddescription of this type of inductor and of its traveling-field andstatic-field modes of operation may moreover be found, if needed, in thePCT international patent application published in the name of theApplicant under No. WO 99/30856.

[0049] In FIG. 3, the bottom position “M” of the magnetic polecorresponds to a maximum current in the winding 16, 16′, associated witha zero current in the winding 17, 17′. Conversely, the top position “N”in FIG. 3 corresponds to a maximum current in the winding 17, 17′associated with a zero current in the winding 16, 16′. Of course, it ispossible to adjust the location of the pole of the inductor to any levelbetween these two extreme positions by combining the intensities of thecurrents using the adjusting means 21 with which the power supply 20 isequipped.

[0050] It may be clearly seen in FIG. 4 that the two matched flatinductors 14 and 15 are configured so that their respective magneticpoles facing each other have opposite polarities. Consequently, themagnetic field of one is added to the magnetic field of the other at anypoint in the gap between the two inductors. The configuration is of the“traversing field” type, as illustrated by the arrows B, the lines offorce joining the magnetic poles of one inductor to the other bycrossing, perpendicularly, the main casting plane P, and therefore thedirection of the streams of molten metal leaving the nozzle.

[0051] Seen from another angle, this same type of configuration is shownagain in FIG. 3. The traversing magnetic field created by the poles ofeach inductor 14, 15 may be shifted vertically by a distance “d” from abottom location “M”, where the magnetic braking action on the flows fromthe main outlets 7 is a maximum, to a top location “N” corresponding toa magnetic braking action which is reduced on the main outlets 7 butincreased on the secondary outlets 8.

[0052] It goes without saying that the invention is not limited to theembodiments exemplified above but extends to many variants orequivalents provided that its definition given in the appended claims issatisfied.

[0053] It will be understood that although the nozzle must have outletsin the main casting plane of the mold in order for the invention to beapplicable, it may also be provided with other outlets placed elsewhere,for example diagonally in the direction of the corners of the mold. Infact, the more the direction of the outflows becomes orthogonal to thefield's lines of force, the more the invention produces its effects,since the effectiveness of the electromagnetic action obtained isdirectly proportional to the vector product of the magnetic field andthe velocity vector of the streams as they leave the outlets of thenozzle.

[0054] Likewise, although the design of the invention has been mainlymotivated with the aim of being able to better manage the heat influx tothe free surface from the actual molten metal arriving in the mold and,consequently, has been preferably aimed at nozzles provided with certainoutlets directed downward and others directed upward, the inventionnevertheless remains of general application to any nozzle whose outletsdo not all have the same direction. This is because as soon as twooutlets have different, even slightly different, directions, for examplediffering by only a few degrees in angle, the invention applies in allstrictness. However, it applies provided that these two outlets are allthe same sufficiently far apart to allow a traversing magnetic field tocover one of them and not the other, or at least to allow it to coverboth of them, but with induction values which, at the same moment, arepalpably different from one another. Thus, as will doubtlessly have beenunderstood, it is the possibility of having a difference in theintensity of the field between two points in the internal space of amold for continuously casting products of elongate shape which is thevery basis of the original concept of the invention.

[0055] Thus, although the invention gives better results in the case of“box”-type nozzles mentioned above, it also applies to straight nozzles,the essential point being that the submerged entry nozzles used for thecasting must have different outlets falling within at least two types bythe directions—usually upward and downward—that they impose on thestreams of molten metal which leave therefrom parallel to the broadfaces. In other words, the invention also applies, for example, tostraight nozzles having lateral outlets differing by being top andbottom over the shaft of the nozzle.

[0056] Moreover, it was implicitly assumed above that the intensity B ofthe magnetic field remains constant. However, as already indicated, itmay very well vary by the intensity of the supply currents beingmodified, the field itself possibly being moved in the gap at the sametime or separately.

[0057] Likewise, as shown in FIG. 5, the inductor 14 (like the inductor15 of course) may be divided into two identical parts 14 a and 14 bplaced side by side on the same face of the mold on each side of thecasting axis S on which the casting nozzle is moreover conventionallycentered. In this way, the lateral regions of the nozzle are “covered”independently of each other by a magnetic field so as to be able to actselectively on the streams of teemed metal 11, 12 leaving these regions.By autonomously adjusting the inductive parts 14 a and 14 b, it is thuspossible to further optimize the symmetry of the flows in the mold asthey are acted upon at the very moment they leave the nozzle. Thisresult, of course, is obtained as a complement to the primary effect ofthe invention which remains the distribution between the various nozzleoutlets of the total outflow of metal by vertically adjusting themagnetic pole on each inductive part 14 a and 14 b. In this version,each inductive part is supplied with current by its own individual powersupply (not shown) so as to be able to adjust, as required, the variousheights of the magnetic pole on each of them and to separately modifythe intensities of the current flowing through them.

[0058] Moreover, instead of inductors of the “traveling field” type, itis possible to opt not only for electromagnets, as already mentioned,but also for permanent magnets, either natural or industrial.

[0059] Furthermore, the expression “individual DC power supplies” usedin the description means not necessarily adding structurally independentindividual power supplies but also a single polyphase power supply,having two or three phases and variable frequency, which are set at zerofrequency in order to obtain a direct current. Polyphase power suppliesof this type are well known. They are of the type comprising an inverterwith a variable chopping threshold and are ordinarily used to actuateelectric motors having a rotating or traveling magnetic field. Theoperation of such a power supply to power the windings of the inductor14, with one phase per winding, consists in adjusting the inverter tothe zero frequency, making such adjustments at chosen times so that theintensities of the currents in each phase are, at these times, thosethat it is desired to obtain in the windings connected to these phases.

[0060] The reader is also reminded that although the preferred field ofapplication of the invention is that of the continuous casting of steelslabs, for which it was moreover initially designed, it neverthelessremains applicable to the continuous casting of metals in general and tothe continuous casting of thin slabs in particular.

1. An apparatus for feeding a mold of a plant for the continuous castingof products of rectangular cross section, such as slabs, with moltenmetal, which comprises: a submerged entry nozzle (6) provided withoutlets for the molten metal which lie in, or substantially in, the maincasting plane (P) parallel to the broad faces of the mold, these outletsdiffering in their direction of outflow and falling within at least twoseparate types (7, 8); an inductive unit (14, 15) placed over the broadfaces of the mold in order to produce thereon magnetic poles of oppositesign facing each other on each side of said main casting plane (P) anddelivering, in its gap substantially surrounding the nozzle (6), atraversing magnetic field covering the outlets of at least one (7) ofsaid types (7, 8); and means (20, 21) for adjusting the relativeintensity of said magnetic field, in the region of the outlets of saidtype (7) which is covered, with respect to the outlets of the other type(8), so as to be able to modify the distribution of the total flow ofmolten metal between all the outlets of said nozzle (6).
 2. Theapparatus as claimed in claim 1, wherein said inductive unit is anelectromagnetic unit consisting of at least one electromagnet.
 3. Theapparatus as claimed in claim 1, wherein said inductive unit consists ofinductors (14, 15) having a plurality of phase windings of the“traveling field” type, facing each other on each side of said maincasting plane (P), and of an associated power supply which supplies eachof said windings separately with DC current and wherein the means (20,21) for adjusting the relative intensity of the magnetic field comprisemeans for moving the location of the magnetic poles in the gap of saidelectromagnetic unit.
 4. The apparatus as claimed in claim 1, whereinsaid inductive unit consists of at least one permanent magnet.
 5. Theapparatus as claimed in claim 2 or 3, wherein said means for adjustingthe relative intensity of the magnetic field comprise a device forvarying the intensity of the electric current supplied to the inductiveunit.
 6. The apparatus as claimed in claim 2 or 4, wherein said meansfor adjusting the relative intensity of said magnetic field comprise anarrangement in which the magnets or electromagnets can move in a slidingmanner.
 7. The apparatus as claimed in claim 3, wherein said means formodifying the location of the magnetic poles in the gap consist of meansfor separately adjusting the intensities of the DC electric currentsindividually supplying the phase windings of said inductors (14, 15). 8.The apparatus as claimed in any one of claims 1 to 7, wherein saidinductive unit consists, on each side of the main casting plane (P), oftwo similar entities (14 a, 14 b) placed side by side on each side ofthe casting axis.
 9. The apparatus as claimed in any one of thepreceding claims, wherein the submerged entry nozzle is a nozzleprovided, in the main casting plane (P), with lower main outlets (7)directed toward the bottom of the mold and with upper secondary outlets(8) directed upward.
 10. The apparatus as claimed in claim 9, whereinthe lower main outlets form one and the same outlet.
 11. A process foroperating an apparatus as claimed in claim 1 so as to feed a mold of aplant for the continuous casting of products of rectangular crosssection with molten metal, wherein the relative intensity of themagnetic field produced by the magnetic poles of the inductive unit isadjusted by moving the position of the magnetic poles.
 12. A process foroperating an apparatus as claimed in claim 1 so as to feed a mold of aplant for the continuous casting of products of rectangular crosssection with molten metal; wherein the relative intensity of themagnetic field produced by the magnetic poles of the inductive unit isadjusted by modifying the intensity of the electric current supplyingsaid inductive unit.